As heat loss in a gas-fired or atmosphere-controlling heating furnace, the most predominant factor is heat brought out by high-temperature exhaust gas that passes through exhaust ports provided in a heat chamber of the heating furnace and becomes radiated to the outside. Accordingly, a method and apparatus for improving thermal efficiency of a heating furnace by and with which the amount of heat flowing out from an exhaust port to the outside is reduced by installing heat-resistant cloth members inside an exhaust port of a heating furnace along the flow of passing exhaust gas to heat the cloth members and by radiating radiation heat inside the heating furnace by means of the heated cloth members are proposed (see e.g., Patent Literature 1).